Making Do With What's Available.

The demons are wrong, of course. The Tree Father is not just stealing the first abyssal plane. He is refining it into his core, and he will do more. They can be forgiven for that mistake because there's little difference between what he is doing and what they claim he is doing for now. 

But they are wrong about his plans for the abyss. He doesn't plan to refine just the first abyssal plane. He plans to refine the whole abyss. That way, his world forge will be very strong. So not only is he stealing the first abyssal plane, he plans to steal every plane in the abyss too. 

As a divine spark, his future is to become a world forge. He needs three things to achieve that. He needs a lot of cosmic energy, a world engine, and a world forge. When he has all three, his divine spark will light the spark in the forge, kickstart the world engine to power the world forge, fuse the two of them together with cosmic energy, and then he will become a maker of worlds.